1. Field of the Invention
This invention relates to the multiplex addressing of bistable liquid crystal displays with greyscale, particularly ferroelectric liquid crystal displays.
2. Discussion of Prior Art
Liquid crystal display devices are well known. They typically comprise a liquid crystal cell formed by a thin layer of a liquid crystal material held between two glass walls. These walls carry transparent electrodes which apply an electric field across the liquid crystal layer to cause a reorientation of the molecules of liquid crystal material. The liquid crystal molecules in many displays adopt one of two states of molecular arrangement. Information is displayed by areas of liquid crystal material in one state contrasting with areas in the other state. One known display is formed as a matrix of pixels or display elements produced at the intersections between column electrodes on one wall and line (or row) electrodes on the other wall. The display is often addressed in a multiplex manner by applying voltages to successive line and column electrodes.
Liquid crystal materials are of three basic types, nematic, cholesteric, and smectic each having a distinctive molecular arrangement.
The present invention concerns ferroelectric smectic liquid crystal materials. Devices using this material form the surface stabilised ferroelectric liquid crystal (SSFLC) device. These devices can show bistability, ie the liquid crystal molecules, more correctly the molecular director, adopt one of two aligned states on switching by positive and negative voltage pulses and remain in the switched state after removal of the voltage. The two states can appear as dark (black) and light (white) areas on a display. This bistable behaviour depends upon the surface alignment properties and chirality of the material.
A characteristic of SSFLCs is that they switch on receipt of a pulse of suitable voltage amplitude and length of time of application, ie pulse width, termed a voltage time product V.t. Thus both amplitude and pulse width need to be considered in designing multiplex addressing schemes.
There are a number of known systems for multiplex addressing ferroelectric displays: see for example article by Harada et al 1985 S.I.D. Paper 8.4 pp 131-134, and Lagerwall et al 1985 I.D.R.C. pp 213-221. See also GB 2,173,336-A, and GB 2,173,629-A. Multiplex addressing schemes for SSFLCs employ a strobe waveform that is applied in sequence to lines but not necessarily to successive lines simultaneously with data waveforms applied to eg column electrodes.
There are two basic types of addressing. One uses two fields of addressing with a first strobe (eg positive strobe) in a first field, followed by a second strobe (eg negative strobe) in a second field; the two fields making up a frame which is the time taken to completely address a display. The other type of addressing uses a blanking pulse to switch all pixels in one or more lines to say a black state, followed by a single strobe pulse applied sequentially to each line for selectively switching pixels in that line to a white state. In this blanking addressing system the frame time is the time required to blank plus the time taken to strobe all the lines.
The bistability property, together with the fast switching speed, makes SSFLC devices suitable for large displays with a large number of pixels or display elements. Such ferroelectric displays are described for example in;- N. A. Clark and S. T. Lagerwall. Applied Physics Letters Vol 36. No 11 pp 889-901. Jun. 1980; GB-2.166.256-A; U.S. Pat. No. 4,367,924; U.S. Pat. No. 4,563,059; patent GB-2,209,610; R. B. Meyer et al. J Phys Lett 36, L69, 1975.
For many displays two visible states only are required, ie an ON state and an OFF state. Examples of such displays include alpha numeric displays and line diagrams. There is now an increasing requirement for a plurality of visible states between the ON and OFF states, ie a plurality of different contrast levels. Such different levels are termed greyscales. Ideally the number of greyscales should be around 256 for good quality pictures, but worthwhile displays can be achieved with much lower values, eg 16 or less.
There are two known techniques for providing greyscale; temporal, and spacial dither. Temporal dither involves switching a pixel to black for a fraction of a frame time and white for the remainder. Providing the switching speed is above a flicker threshold (eg above about 35 Hz), a user's eye integrates over a period of time and sees an intermediate grey whose value depends upon the ratio of black to white time. Spatial dither involves dividing each pixel into individually switchable subpixels which may be of different size; each subpixel is sufficiently small at normal viewing distances that subpixels can not be distinguished individually. Both temporal and spacial dither techniques can be combined to increase the number of greyscale levels in a display; see EP9000942, 0453033, W. Hartmann, J. van Haaren.
Patent specification EP-0,214,857 describes a ferroelectric liquid crystal display with greyscale. Greyscale display is achieved by addressing each line of display with three successive equal period frame times, applying a scanning voltage at the beginning of each frame and blanking once per frame at a different time position within the three frames (other specifications would describe these three frames as three fields making up a single frame time). This gives a display with three different time periods when the display can be in a light state; these together with an all dark state gives eight different levels of greyscale. One disadvantage with this arrangement is a low maximum light intensity from the display.
Patent specification EP-261,901 describes a ferroelectric liquid crystal display with greyscale. The time to address a complete display, namely a frame time, is divided into fields of different lengths, hence a pixel can be switched into a light or a dark state for a time approximately equal to the length of each field. Each line is completely addressed in one frame time. A line is addressed (switched to an ON or OFF state) at the start (for a particular line) of each field time. To obtain a binary increase in greyscale levels the length of each field would increase in binary manner. For any reasonable number of lines to be addressed it is not possible to increase the length of each field in the desired progression in order to achieve a desired separation between the different levels of greyscale.
Patent Specification GB-A-2164776 is similar to EP-261,901 in having different length field times within a frame time. Pixels can be either light or dark in each field time. Thus a total of six different levels of greyscale are obtainable from 3 different length field times.
Patent Specification EP-A--0306011 describes a driving method for matrix of column and row electrodes in a ferroelectric liquid crystal display. A frame time is divided into three unequal length field times. The driving method comprises: dividing, the column electrodes into K groups of column electrodes, defining the number Z of column electrode lines constituting each group of the column electrodes, rendering one frame period, selecting a predetermined one of the K groups of the column electrodes for a time width ZTo of each of the blocks so that each picture element on the selected one of the groups of the column electrodes can be set in one of the bright and dark memory states; and selecting a number of times not smaller than n the K groups of thecolumn electrodes during each one-frame period T.sub.F according to a predetermined sequence.
One problem with existing addressing systems is that of providing different greyscale levels that are suitably different in intensity, and with a high overall display brightness.
Even with a combination of temporal and special dither it is still difficult to provide a suitable spacing of greyscale levels.